Structure and method for asymmetric waveguide nitride laser diode

ABSTRACT

A structure and method for an asymmetric waveguide nitride laser diode without need of a p-type waveguide is disclosed. The need for a high aluminum tunnel barrier layer in the laser is avoided.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Agreement No.MDA972-96-3-0014 awarded by DARPA. The Government may have certainrights in the invention.

BACKGROUND

The present invention relates generally to the field of laser diodes,and more particularly to short-wavelength nitride based laser diodes.Short-wavelength nitride based laser diodes provide smaller spot sizeand a better depth of focus than red and infrared (IR) laser diodes forlaser printing operations and other applications. Single-spot nitridelaser diodes have applications in areas such as high density-opticalstorage.

Laser diodes based on higher bandgap semiconductor alloys such asAlGaInN have been developed. Excellent semiconductor lasercharacteristics have been established in the near-UV to violet spectrum,principally by Nichia Chemical Company of Japan. See for example, S.Nakamura et al., “CW Operation of InGaN/GaN/AlGaN-based laser diodesgrown on GaN substrates”, Applied Physics Letters, Vol. 72(6), 2014(1998), S. Nakamura and G. Fasol, “The Blue Laser Diode-GaN based LightEmitters and Lasers”, (Springer-Verlag, 1997) and also A. Kuramata etal., “Room-temperature CW operation of InGaN Laser Diodes with aVertical Conducting Structure on SiC Substrate”, Japanese Journal ofApplied Physics, Vol. 37, L1373 (1998), all of which are incorporated byreference in their entirety.

For laser diodes and arrays incorporated into printing systems,reliable, low threshold operation is a basic requirement. Among thedifficulties associated with achieving low threshold operation is theconfinement of injected electrons in the quantum well active region. Ifthe injected electrons are not appropriately confined, the electronsleak away from the quantum well active region and recombine with theholes injected from the p-layers of the structure. For example, in thenitride laser structure pioneered by S. Nakamura of Nichia ChemicalCompany, a thin, high bandgap layer is placed immediately adjacent tothe active region to confine the injected electrons. In the Nakamurastructure, a 200 Å layer of Al_(0.2)Ga_(0.8)N:Mg acts as a tunnelbarrier layer to prevent the energetic electrons (electrons havingsufficient energy to escape from the quantum wells) from diffusing intothe p-type material, where recombination with the available holes wouldoccur. Reducing electron leakage lowers the laser threshold current andits temperature sensitivity while raising the quantum efficiency of thelaser.

FIG. 1 shows conventional nitride laser structure 100. Conventionalnitride laser structure 100 uses both GaN:Mg p-waveguide layer 115 andGaN:Si n-waveguide layer 116 with Al_(0.2)Ga_(0.8)N:Mg tunnel barrierlayer 110 positioned over In_(0.12)Ga_(0.88)N/In_(0.02)Ga_(0.98)N:Simultiple quantum well active region 120. Al_(0.07)Ga_(0.93)N:Mgp-cladding layer 130 is positioned over p-waveguide layer 115 whileAl_(0.07)Ga_(0.93)N:Si n-cladding layer 131 is positioned belown-waveguide layer 116. GaN:Mg layer 140 serves as a capping layer tofacilitate ohmic contact while Al₂O₃ layer 150 serves as the growthsubstrate. An Ni/Au p-contact (not shown) on top of GaN:Mg layer 140, aTi/Al contact (not shown) on exposed surface of GaN:Si layer 155. GaN:Silayer 155 is a lateral contact layer while In_(0.03)Ga_(0.97)N:Si layer156 is the defect reduction layer to prevent defect propagation. GaNlayer 160 functions as a buffer layer.

FIG. 2 illustrates the function of tunnel barrier layer 110 using asimplified band diagram. Tunnel barrier layer 110 is a p-type AlGaNlayer which acts as a strong confinement barrier for injected electrons.Quantum wells 220, 221, 222, 223 and 224 comprising active region 120are InGaN while tunnel barrier layer 110 is AlGaN. The potential energylevel 250 for the conduction band electrons and quasi-fermi level 255are shown for AlGaN tunnel barrier layer 110 with low p-doping energylevel 230 and high p-doping energy level 240 are shown with respect topotential energy level 250 for electrons and fermi level 255 for theconduction band. Quasi-fermi level 260 for the holes is shown along withpotential energy level 265 for holes. Successful operation of Nakamuratype laser structures requires successful p-type doping of high-bandgapAlGaN tunnel barrier layer 110. However, the growth of tunnel barrierlayer 110 presents many practical difficulties, including the difficultyof p-doping high aluminum content alloys and the difficulty of reliablygrowing high aluminum content alloys because of parasitic pre-reactionsbetween trimethylaluminum and ammonia during metalorganic chemical vapordeposition (MOCVD). If the hole concentration or aluminum content intunnel barrier layer 110 is insufficient, the ability of layer 110 tocontain electrons is reduced since electron confinement increases withthe p-type doping level.

P-cladding layer 130 can be used to confine injected electrons in anitride laser diodes if it is placed in close proximity, typicallywithin 1 minority carrier diffusion length, to the multiple-quantum wellactive region. A difficulty with this approach is that multiple-quantumwell active region 120 is typically located at the core of a waveguideregion to maximize the spatial overlap with the optical mode as shown inFIG. 3 for conventional nitride laser diode structure 100. However, thisplaces p-cladding layer more than 1 minority carrier diffusion lengthfrom multiple-quantum well region 120. Refractive index profile 310 andcorresponding fundamental transverse optical mode 320 are shown as afunction of distance relative to the interface between n-cladding layer131 and n-waveguide layer 116. The waveguide thickness is adjustedindependently to maximize the optical confinement factor, Γ. Opticalconfinement factor, Γ is the fraction of the power that overlapsmultiple-quantum well active region 120 where the optical gain isgenerated. For nitride laser diodes, the typical thickness for thewaveguide above and below multiple-quantum well active region 120 isabout 100 nm which is greater than 1 electron diffusion length. Thisplaces p-cladding layer 130 in conventional nitride laser diodestructure 100 to far away from multiple-quantum well active region 120to confine the injected electrons.

SUMMARY OF INVENTION

In accordance with the present invention, a p-type cladding layer isused to eliminate the p-type waveguide and eliminate the need for ap-type, very high bandgap, high-aluminum content AlGaN tunnel barrierlayer in nitride laser diodes. The p-type cladding layer is used tosuppress electron leakage. In addition to the p-type cladding layer, ahigh-Al-content tunnel barrier, a superlattice structure or adistributed electron reflector may be placed at the multiple quantumwell region/p-cladding layer interface. Although a p-type cladding layeris used for suppressing electron leakage in laser diodes fabricated fromother materials such as arsenides and phosphides, the use of p-claddinglayer in nitride laser diodes is not straightforward. The minoritycarrier diffusion lengths (average distance minority carrier travelsbefore recombination occurs) in nitrides are many times shorter than inother laser diode materials. Hence, the p-cladding layer typically liesseveral diffusion lengths away from the multiple-quantum well activeregion. Consequently, injected electrons are not appreciably confined bythe p-cladding layer which leads to the requirement for thehigh-aluminum content tunnel barrier layer. In red and infrared laserdiodes, the waveguide thickness is a mere fraction of the diffusionlength, so that the cladding layer can effectively suppress leakage, seefor example, “Drift leakage current in AlGaInP quantum well laserdiodes, “D. P. Bour, D. W. Treat, R. L. Thomton, R. S. Geels, and D. F.Welch, IEEE Journal of Quantum Electronics, vol. 29, pp. 1337-1343(1993).

A high optical confinement factor can still be achieved for nitridelaser diode structures if a p-cladding layer is positioned adjacent tothe multiple-quantum well active region instead of the typical 100 nmdistance away which maximizes the optical confinement factor. This isdue to the relatively weak transverse (perpendicular to the layerplanes) waveguiding that occurs in nitride lasers which results in muchof the mode spreading evanescently into the cladding layers. Indeed, therefractive index difference, Δn, between the waveguide core and thecladding layers is only about 0.05 which is nearly one order ofmagnitude less than that in typical AlGaAs lasers. The weak transversewaveguiding results in a less strongly peaked waveguide mode which makesthe optical confinement factor less sensitive to any wave guideasymmetry.

A superlattice may be introduced into the asymmetric waveguide nitridelaser diode structure or a conventional nitride laser structure toenhance carrier confinement. The superlattice is used to replace auniform bulk layer. A properly designed superlattice inhibits thetendency for structural defect formation while allowing adequate p-typedoping and carrier confinement in the quantum wells. For example, asuperlattice that alternates GaN with AlGaN layers allows high p-dopingsince the GaN layers are readily p-doped. Carrier confinement requiresadequate band offsets in the valence and conduction bands between thequantum well active region and the surrounding layers. Carrierconfinement by superlattice structures also requires avoiding resonanttunneling effects.

Short period superlattices may be designed to act as coherent electronreflectors. Short period superlattices function as distributed Braggreflectors which reflect the wavefunction of leaked electrons back intothe multiple quantum well active region. Similar structures, oftencalled “Multi-Quantum Barriers” are used to confine electrons in shortwavelength (λ<650 nm) red AlGaNInP lasers where they are placed in thep-cladding layer rather than immediately next to the multiple quantumwell active region. As coherent reflections may be produced usinglow-bandgap superlattice layers, the need for AlGaN layers may bereduced or eliminated. This preserves the structural quality of filmswhile transverse waveguiding is not negatively effected by AlGaN layersand p-type doping benefits from the ability to use low-bandgap barrierlayers. The thickness of the layers making up the superlattices needs tobe selected to avoid resonant tunneling. Appropriate selection of layerthicknesses allows an electron reflectivity of about 100% for electronenergies beyond the classical barrier height. Therefore, properlydesigned distributed electron reflectors may be more effective than bulkbarrier layers for confining injected electrons.

Hence, in accordance with the present invention, nitride laser diodestructures can be made which eliminate the need for the p-type waveguidelayer and the high-aluminum-content tunnel barrier and have a p-claddinglayer deposited above the multiple quantum well active region to confineelectrons. Additionally, superlattices may be introduced between themultiple quantum well region and the p-cladding layer to enhance carrierconfinement.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained and understood by referringto the following detailed description and the accompanying drawings. Thedrawings, briefly described below, are not to scale.

FIG. 1 shows a prior art nitride laser diode structure.

FIG. 2 shows a band diagram for a conventional nitride laser diodestructure.

FIG. 3 shows the refractive index profile and corresponding fundamentaltransverse optical mode for a conventional nitride laser diode.

FIG. 4a shows an asymmetric waveguide nitride laser diode structure inaccordance with the invention.

FIG. 4b shows a band diagram for an asymmetric waveguide laser diodestructure.

FIG. 5 shows the refractive index profile and corresponding fundamentaltransverse optical mode for an asymmetric waveguide nitride laser diodein accordance with the invention.

FIG. 6 shows a comparison between a conventional nitride laser diode andan asymmetric waveguide nitride laser diode in accordance with theinvention.

FIG. 7 shows a comparison between a conventional nitride laser diode andasymmetric waveguide nitride laser diodes in accordance with theinvention.

FIG. 8 shows a comparison between the total optical confinement factorsof two asymmetric waveguide nitride laser diodes in accordance with theinvention.

FIG. 9a shows an asymmetric waveguide nitride laser diode structure inaccordance with the invention.

FIG. 9b shows a band diagram fro an asymmetric waveguide laser diodestructure in accordance with the invention.

FIG. 10 shows a superlattice structure in accordance with the invention.

FIG. 11 shows a superlattice structure in accordance with the invention.

FIG. 12 shows an electron reflection spectrum for the superlattice inFIG. 11.

FIG. 13 shows a superlattice structure in accordance with the invention.

FIG. 14 shows an electron reflection spectrum for the superlattice inFIG. 13.

FIG. 15 shows a superlattice structure in accordance with the invention.

FIG. 16 shows an electron reflection spectrum for the superlattice inFIG. 15.

FIG. 17 shows a quartz reactor.

FIGS. 18(a) and 18(b) show tables for the parameters and the sequence oflayer deposition in accordance with the invention.

DETAILED DESCRIPTION

In accordance with an embodiment of the present invention, FIG. 4a showsasymmetric waveguide nitride laser diode structure 400 with nop-waveguide layer 115 and no AlGaN tunnel barrier layer 110 overmultiple quantum well region 120. P-cladding layer 130 is positionedadjacent multiple quantum well active region 120 with thin undoped GaNlayer 429 acting as a transition layer between the two. Substrate 150,may be, for example, composed of Al₂O₃ but may be SiC or GaN or othersuitable substrate material. Typical values for pulsed threshold currentdensity for laser diode structure 400 are 5 kA/cm² at 6.5 volts witharea dimensions of about 750 μm by 3 μm.

FIG. 4b shows a band diagram of the central region of laser diodestructure 400. Note the difference with the band diagram forconventional structure 100 in FIG. 2. In conventional structure 100,p-waveguide layer 115 and tunnel barrier layer 110 are present betweenmultiple-quantum well active region 120 and p-cladding layer 130. Thethickness of p-waveguide layer 115 is approximately equal to thethickness of n-waveguide layer 116. In asymmetric waveguide nitridelaser diode structure 400, p-waveguide layer 115 is eliminated andp-cladding layer 130 is located close to multiple-quantum well activeregion 120 with only undoped GaN transition layer 429. In asymmetricstructure 400, p-cladding layer 130 functions as both a cladding layerfor optical confinement and an electronic confinement barrier forinjected electrons.

FIG. 5 shows modeled refractive index profile 510 and correspondingfundamental transverse optical mode profile 520 for asymmetric waveguidenitride laser diode structure 400 where tunnel barrier layer 110 iseliminated. MODEIG dielectric waveguide simulation software was used forthe modeling as in FIG. 3. The software may be downloaded from the website: www.seas.smu.edu/modeig. FIG. 5 shows that for asymmetricwaveguide nitride laser diode structure 400, mode peak 550 is notcoincident with multiple quantum well region 120 position 530. In FIG.6, Curve 610 represents Γ_(total) for conventional nitride laser diodestructure 100 while curve 620 represents Γ_(total) for asymmetricwaveguide nitride laser diode structure 400. FIG. 6 shows that therelative displacement of mode peak 550 (see FIG. 5) frommultiple-quantum well active region 120 position 530 does notappreciably degrade optical confinement factor Γ. This is the casebecause the mode is very weakly confined in the case of nitride laserdiodes. Γ_(total) values in FIG. 6 represent the sum of the individualΓ's for each of five quantum wells in quantum well active region 120 andare plotted versus the thickness of n-waveguide layer 116. Conventionalnitride laser structure 100 is taken to have tunnel barrier layer 110separated from p-cladding layer 130 by 100nm thick p-waveguide 115.Although Γ_(total) is slightly higher for conventional nitride laserstructure 100, Γ_(total) is nominally still about 5% for bothconventional nitride laser structure 100 and asymmetric waveguidenitride laser structure 400.

FIG. 5 shows that a slight displacement of multiple-quantum well activeregion 120 position 530 toward mode peak 550 and away from p-claddinglayer 130 will result in Γ_(total) values that are higher. Increasedseparation between p-cladding layer 130 and multiple-quantum well activeregion 120 to about 20 nm in asymmetric waveguide nitride laserstructure 400 may be accomplished by increasing the thickness of GaNtransition layer 429 to achieve an optical confinement factor that issomewhat greater than that achieved in conventional nitride laserstructure 100 as seen in FIG. 7 The added portion of GaN transitionlayer 429 may be p-doped. FIG. 7 compares Γ_(total) for conventionalnitride laser structure 100 with two embodiments in accordance with thepresent invention. Curve 710 shows the total optical confinement factorfor conventional nitride laser diode structure 100 having 100 nmp-waveguide 115 with 10 nm tunnel barrier layer 110. Calculated curves720 and 730 show structure 400 with a typical 6 nm and an increased 20nm separation, respectively, between p-cladding layer 130 and multiplequantum well active region 120. However, increasing the separationbetween multiple quantum well region 120 and p-cladding layer 130 maysignificantly reduce the electrical confinement provided by p-claddinglayer 130 due to the short minority carrier diffusion length in nitridematerials. Hence, the improved optical confinement may be more thanoffset by the reduced confinement of injected electrons.

In an embodiment in accordance with the present invention, thin(typically about 2-6 nm thickness) undoped GaN layer 429 is insertedbetween multiple-quantum well region 120 and p-cladding layer 130. GaNlayer 429 is deposited at a slow rate (about equal to the rate used formultiple quantum well region 120) while reactor conditions are changedfrom conditions that are optimal for growth of multiple quantum wellregion 120 to those of p-cladding layer 130. Undoped GaN layer 429 is atransition layer which accommodates the difference in growth conditionsbetween multiple quantum well active region 120 and 130. Specifically,to incorporate indium (In) into multiple quantum well active region 120requires a temperature of about 775° C. with no hydrogen carrier gasflow. To achieve better uniformity and allow abrupt gas switching at theinterfaces between the individual quantum wells of multiple quantum wellactive region, a low pressure (about 200 Torr) environment is used. Lowpressure growth of the quantum wells also allows the hydrogen carriergas flows through the organometallic bubbler sources to be minimized.The deposition of p-cladding layer 130 requires considerably differentparameters than those for active region 120. High hydrogen carrier gasflows of 10 slpm (standard liters per minute) are required to inhibitpre-reactions between trimethylaluminum and ammonia. Similarly, forp-doping a high pressure (about 700 Torr) and a temperature of 900° C.is used to achieve good quality p-cladding layer 130.

In an embodiment in accordance with the present invention, total opticalconfinement factor Γ_(total) may be increased by increasing the aluminumcontent of n-cladding layer 131. FIG. 8 shows how total opticalconfinement factor Γ_(total) increases when the composition ofn-cladding layer 131 is changed from Al_(0.07)Ga_(0.93)N shown by curve810 to Al_(0.10)Ga_(0.90)N shown by curve 820. Hence, an asymmetry inthe composition of p-cladding layer 130 and n-cladding layer 131 can beused to compensate for the displacement of multiple quantum well activeregion relative to fundamental transverse optical mode peak 510 tomaintain high total optical confinement factor Γ_(total).

FIG. 9a shows modified asymmetric waveguide nitride laser diodestructure 900 for providing enhanced electron confinement. Asymmetricwaveguide nitride laser diode structure 900 has added layer 910 at theinterface between multiple quantum well region 120 and p-cladding layer130. Added layer 910 may be n-period superlattice 910 a, distributedelectron reflector 910 b or high aluminum content tunnel barrier layer910 c.

FIG. 9b shows a band diagram of the central region of asymmetricwaveguide nitride laser diode structure 900. FIG. 9b is similar to FIG.4b except for the insertion of added layer 910 which is an additionalelectronic confinement layer between multiple-quantum well active region120 and p-cladding layer 130. In FIG. 9b, the band edges in added layer910 are shown by dotted lines; indeed, layer 910 may comprise ann-period supperlattice.

FIG. 10 shows 5-period superlattice 910 a. Superlattice 910 a consistsof Al_(x)Ga_(1−x1)N layers 1051, 1053, 1055, 1057 and 1059 withthickness d₁ and Al_(x2)Ga_(1−x2)N layers 1052, 1054, 1056, 1058 and1060 with thickness d₂. A typical choice for superlattice 910 a is tochoose compositions and thicknesses so that the average composition,x_(avg), in superlattice 910 a, defined as equal to(x1·d1+x2·d2)/(d1+d2) is equal to the composition of high aluminumcontent tunnel barrier layer 910 c. A lower value of x_(avg) issufficient, however, because barrier layers 1052, 1054, 1056, 1058 and1060 in superlattice 910 a will have a band gap larger than the band gapof high aluminum content tunnel barrier layer 910 c with compositionx_(avg) and quantum confinement in well layers 1051, 1053, 1055, 1057and 1059 will shift the allowed energies for electrons to higher values.To replace uniform Al_(0.2)Ga_(0.8)N:Mg tunnel barrier layer 910 c,superlattice structure 910 a could be selected with x1=0 and x2=0.25.

x1 is selected to allow layers 1051, 1053, 1055, 1057 and 1059 to beadequately p-doped which restricts x1 to values below about 0.1, see forexample, D. Bremser, W. G. Perry, T. Zheleva, N. V. Edwards, O. H. Nam,N. Parikh, D. E. Aspnes, and R. F. Davis, MRS Internet J. NitrideSemicond. Res. 1, 8 (1996) incorporated by reference in its entirety. Atypical value for x1 is to take x1=0, resulting in GaN layers 1051,1053, 1055,1057 and 1059. x2 is selected to provide a high enough bandgap for superlattice 910 a to enable effective electron confinement witha typical value for x2 being x2=0.25 as discussed above. Thicknesses d1and d2 may range between 0 and about 50 Å, with a typical value beingd1=d2=20 Å. A thickness less than about 50 Å is necessary to enablesignificant overlap of electron wavefunctions between well layers 1051,1053, 1055, 1057 and 1059 and barrier layers 1052, 1054, 1056, 1058 and1060. A barrier thickness greater than 10 Å allows adequate control overthe growth, with the objective of achieving sharp interfaces and highdoping in well layers 1051, 1053, 1055, 1057 and 1059. Based oncalculations of wavefunctions, thicknesses d1 and d2 between 10 and 20 Åare optimal. This results in a total thickness for superlattice 910 a ofbetween 100 and 200 Å. The lowest allowed energy for electrons insuperlattice 910 a is at a level between the conduction band in thequantum wells of superlattice 910 a and the conduction band in thequantum barriers of superlattice 910 a. The lowest allowed energy forelectrons increases as the thickness of the well decreases. Use ofsuperlattice 910 a also reduces the tendency for structural defectformation and improves the ability to accomplish p-type doping of thetunnel barrier. Superlattice 910 a may also be used in place of tunnelbarrier layer 110 in conventional nitride laser diode structure 100.

FIG. 11 shows 5 pair short period Al_(0.2)Ga_(0.8)N/GaN superlatticestructure 1100 in an embodiment of distributed electron reflector 910 bin accordance with the present invention. GaN layers 1102, 1104, 1106,1108 and 1110 are selected to have a thickness of approximately 2 nmwhile AlGaN layers 1101, 1103, 1105, 1107, 1109 are selected to have athickness of approximately 3 nm. The respective layer thicknesses areselected in part to prevent resonant tunneling by injected electrons atenergies below about 700 meV while producing strong reflection up to 700meV. For example, FIG. 12 shows the reflectivity versus electron energyfor superlattice structure 1100 where zero meV corresponds to theconduction-band position in quantum wells in multiple-quantum wellactive region 120, and 300 meV to the conduction-band position in thebarrier layers of multiple-quantum well region 120. 700 meV is taken asthe conduction-band position in AlGaN barrier layers 1101,1103,1105,1107and 1109.

FIG. 13 shows 5 pair short period InGaN/Al_(0.2)Ga_(0.8)N superlatticestructure 1300 in an embodiment of a distributed electron reflector 910b in accordance with the present invention. Using InGaN instead of GaNallows improved p doping since the acceptor ionization energy is lowerin InGaN. Furthermore, because of the high refractive index of InGaN, acombination of InGaN and AlGaN multiple quantum barrier has an averagerefractive index closer to that of GaN. Hence, structure 1300 isrelatively neutral with respect to transverse waveguiding. In contrast,the typical 200 Å Al_(0.2)Ga_(0.8)N bulk barrier layer has a negativeinfluence on the quantum well active region optical confinement factor,Γ. Finally, InGaN/AlGaN short period superlattice structure 1300provides a form of strain balancing which may suppress the formation ofstructural defects. Strain balancing occurs because InGaN experiencesbiaxial compression while AlGaN experiences biaxial tension insuperlattice structure 1300.

Low-bandgap InGaN layers 1302, 1304, 1306, 1308 and 1310 are selected tohave a thickness of approximately 1 nm while AlGaN layers 1301, 1303,1305, 1307 and 1309 layers are selected to have a thickness ofapproximately 3 nm. The respective layer thicknesses are selected inpart to prevent resonant tunneling by injected electrons at energiesbelow 800 meV while producing strong reflection up to 800 meV. Forexample, FIG. 14 shows the reflectivity versus electron energy forsuperlattice structure 1300 where zero meV corresponds to theconduction-band position in quantum wells in multiple-quantum wellactive region 120, and 300 meV to the conduction-band position in thebarrier layers of multiple-quantum well region 120. 700 meV is taken asthe conduction-band position in AlGaN barrier layers 1301,1303,1305,1307 and 1309. FIG. 14 shows that the effective barrier height ofsuperlattice structure 1300 is greater than the classical barrier heightof 700 meV.

FIG. 15 shows 5 pair short period InGaN/GaN superlattice structure 1500in an embodiment of a distributed electron reflector 910 b in accordancewith the present invention. Superlattice structure 1500 contains nohigh-bandgap AlGaN layers. GaN layers 1502, 1504, 1506, 1508 and 1510are selected to have a thickness of approximately 2 nm while InGaNlayers 1501, 1503, 1505, 1507 and 1509 are selected to have a thicknessof approximately 1 nm. The respective layer thicknesses are selected inpart to avoid resonant tunneling. For example, FIG. 16 shows shows thereflectivity versus electron energy for superlattice structure 1500where zero meV corresponds to the conduction-band position in thequantum wells of multiple quantum well active region 120 and 300 meV tothe conduction-band position in the barrier layers of multiple-quantumwell active region 120. FIG. 16 shows the effective barrier height ofsuperlattice structure 1500 is about 500 meV which is significantlygreater than the classical barrier height of 300 meV. However, FIG. 16shows that superlattice structure 1500 has a narrow transmissionresonance at about 300 meV. Other than the narrow transmissionresonance, superlattice structure 1500 provides a high reflectivity outto about 500 meV. In addition to the benefit of improved p doping andstructural quality, an AlGaN-free multiple quantum barrier such assuperlattice structure 1500 also provides a barrier which improvestransverse waveguiding.

If InGaN layers 1302, 1304, 1306, 1308, and 1310 or 1502, 1504, 1506,1508 and 1510 have a bandgap energy comparable to the multiple quantumwell active region bandgap energy, superlattice structure 1300 or 1500,respectively, should be displaced from multiple quantum well region 120by some minimum distance, d_(min). This displacement inhibits tunnelingof injected electrons from multiple quantum well active region 120 toInGaN layers 1302, 1304, 1306, 1308, and 1310 or 1502, 1504, 1506, 1508and 1510 of superlattice structure 1300 or 1500, respectively. d_(min)is limited by the requirement that the displacement not be so great thatthe electron wavefunction loses coherence before appreciable interactionoccurs with superlattice structure 1300 or 1500. A typical range ford_(min) is about 5-10 nm.

Table 1 in FIG. 18 shows the layers in sequence of deposition onsapphire substrate 150 for asymmetric waveguide nitride laser diodestructure 400 along with the approximate deposition parameters for eachlayer. Organometallic flows are expressed in μmoles/min, calculatedbased on complete saturation of the solid's (TMIn and Cp₂Mg) or liquid's(TMGa, TMAl, and TEGa) vapor by a hydrogen carrier gas. Asymmetricwaveguide nitride laser diode structure 400 is made using metalorganicchemical vapor deposition (MOCVD). FIG. 17 shows quartz reactor cell1700 has an inner diameter of about 80 mm to accommodate a single 5 cmsapphire substrate wafer. The reactor geometry is vertical flow withreactant gases being injected at the top of the reactor through line1730, which is about 25 cm above sapphire wafer surface 1750. Diffuserstructure 1780 is attached to line 1730. To avoid parasiticpre-reactions of the gaseous precursors, the ammonia flow is separatedfrom the alkyl group-III precursors by passing through inlet 1720. Theorganometallic flow is introduced through inlet 1710. Sapphire substratewafer 150 sits on rotating (about 10 rpm) SiC coated graphite susceptor1760. Susceptor 1760 is inductively heated and temperature of susceptor1760 is measured by a fiber-optic-coupled pyrometer (not shown) andcontrolled by a proportional-integral-derivative controller. Thepressure in quartz reactor 1700 may be adjusted between about 50-740Torr by means of a throttle valve (not shown) in the reactor exhaustline (not shown).

With reference to tables 1 and 2 in FIG. 18 and FIGS. 4 and 9 in thefollowing description, sapphire (Al₂O₃) substrate 150 is either a C-face(0001) or A-face (1120) oriented sapphire. Sapphire substrate wafer 150is obtained from Bicron Crystal Products of Washougal, Wash., withstandard specifications including an epitaxial polish on one side and a13 mil thickness. A heat clean is performed on sapphire substrate 150for 600 seconds at a temperature of 1050° C. and pressure of 200 Torrwith an H₂ flow of 10 slpm (standard liters per minute). GaN bufferlayer 160 of 0.03 μm thickness is deposited at a temperature of 550° C.,a pressure of 200 Torr with a TMGa (Trimethylgallium (CH3)₃Ga ) flow of34 μmole/min, NH₃ flow of 4 slpm and H₂ flow of 10 slpm for 120 sec. Anundoped GaN layer (not shown in FIGS.) of 2 μm thickness is deposited ata temperature of 1125° C., a pressure of 700 Torr with a TMGa flow of136 μmole/min, NH₃ flow of 4 slpm and H₂ flow of 10 slpm for 1200 sec.

GaN:Si lateral contact layer 155 of 5 μm thickness is deposited at atemperature of 1100° C., a pressure of 200 Torr with a TMGa flow of 136μmole/min, SiH₄ flow of 0.0002 sccm (standard cubic centimeters perminute), NH₃ flow of 4 slpm and H₂ flow of 10 slpm for 3000 sec.In_(0.03)Ga_(0.97)N:Si defect reduction layer 156 of 0.1 μm thickness isdeposited at a temperature of 800° C., a pressure of 200 Torr with aTEGa (triethylgallium (C₂H₅)₃Ga) flow of 12 μmole/min, a TMIn(trimethylindium (CH₃)₃In) flow of 6 μmole/min, an SiH₄ flow of 0.00002sccm and an NH₃ flow of 5 slpm for 600 sec.

Al_(0.07)Ga_(0.93)N:Si cladding layer 131 of 0.7 μm thickness isdeposited at a temperature of 1100° C., a pressure of 200 Torr, with aTMGa flow of 102 μmole/min, a TMAl (trimethylaluminum (CH₃)₃Al) flow of10 μmole/min, an SiH₄ flow of 0.0002 sccm, an NH₃ flow of 4 slpm and anH₂ flow of 10 slpm for 800 sec. GaN:Si waveguide layer 116 of 0.1 μmthickness is deposited at a temperature of 1100° C., a pressure of 200Torr with a TMGa flow of 34 μmole/min, an SiH₄ flow of 0.00002 sccm, anNH₃ flow of 4 slpm and an H₂ flow of 10 slpm for 300 sec.

The first In_(0.12)Ga_(0.88)N quantum well layer (not shown in FIGS.) ofmultiple quantum well active region 120 with 0.003 μm thickness isdeposited at a temperature of 775° C., a pressure of 200 Torr, with aTEGa flow of 5 μmole/min, a TMIn flow of 24 μmole/min and an NH₃ flow of4 slpm. The first In_(0.02)Ga_(0.98)N:Si barrier layer (not shown inFIGS.) is subsequently deposited on the first quantum well layer.In_(0.02)Ga_(0.98)N:Si barrier layer with 0.006 μm thickness isdeposited at a temperature of 775° C., a pressure of 200 Torr, with aTEGa flow of 5 μmole/min, a TMIn flow of 3 μmole/min, an SiH₄ flow of0.00001 sccm and an NH₃ flow of 5 slpm for 180 sec. This combination ofquantum well layer and barrier layer is repeated three more times underthe same conditions as can be seen from table 1 in FIG. 18. A fifthquantum well layer is then deposited as described in table 1.Subsequently, undoped GaN transition layer 429 with 0.005 μm thicknessis deposited for 150 sec at a temperature that is varied from 775 to900° C. while pressure is varied from 200 to 700 Torr. For GaN layer429, TEGa flow is 5 μmole/min, NH₃ flow is 4 slpm and H₂ flow is 10 slpmfor 150 sec.

Al_(0.07)Ga_(0.93)N:Mg cladding layer 130 with 0.5 μm thickness isdeposited at a temperature of 900° C., a pressure of 700 Torr, with aTMGa flow rate of 34 μmole/min, a TMAl flow rate of 12 μmole/min, aCp₂Mg (biscyclopentadienylmagnesium (C₂H₅)₂Mg) flow rate of 0.5μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate of 10 slpm for2400 sec. GaN:Mg capping layer 140 with 0.1 μm thickness is deposited ata temperature of 900° C., a pressure of 700 Torr, with a TMGa flow rateof 34 μmole/min, a Cp₂Mg flow rate of 0.5 μmole/min, an NH₃ flow rate of4 slpm and an H₂flow rate of 10 slpm for 300 sec.

With reference to table 2 in FIG. 18, and FIG. 9a, 5 pair periodsuperlattice structure 910 a may be deposited immediately on top of GaNtransition layer 429 if desired. Expected conditions for depositingGaN:Mg layer 1051 with 0.002 μm thickness are a temperature of 900° C.,a pressure of 700 Torr with a TMGa flow of 34 μmole/min, a Cp₂Mg flowrate of 0.5 μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate ofslpm for 6 sec. Subsequently a stop is implemented for 5 sec. without achange in temperature or pressure and an NH₃ flow rate of 4 slpm and anH₂ flow rate of 10 slpm. Expected conditions for depositingAl_(0.25)Ga_(0.75)N:Mg layer 1052 with a 0.002 μm thickness are atemperature of 900° C., a pressure of 700 Torr with a TMGa flow rate of34 μmole/min, a TMAl flow rate of 60 μmole/min, a Cp₂Mg flow rate of 0.5μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate of slpm for 6sec. Subsequently a stop is implemented for 5 sec. without a change intemperature or pressure and an NH₃ flow rate of 4 slpm and an H₂ flowrate of 10 slpm. The preceding sequence is repeated 4 times to make 5pair period superlattice 910 a.

With reference to table 2 in FIG. 18, and FIGS. 9a and 11, 5 pair shortperiod Al_(0.2)Ga_(0.8)N/GaN superlattice structure 1100 for distributedelectron reflector 910 b may be may be deposited immediately on top ofGaN transition layer 429 if desired. Expected conditions for depositingGaN:Mg layer 1101 with 0.002 μm thickness are a temperature of 900° C.,a pressure of 700 Torr with a TMGa flow of 34 μmole/min, a Cp₂Mg flowrate of 0.5 μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate ofslpm for 6 sec. Subsequently a stop is implemented for 5 sec. without achange in temperature or pressure and an NH₃ flow rate of 4 slpm and anH₂ flow rate of 10 slpm. Expected conditions for depositingAl_(0.2)Ga_(0.8)N:Mg layer 1102 with a 0.003 μm thickness are atemperature of 900° C., a pressure of 700 Torr with a TMGa flow rate of34 μmole/min, a TMAl flow rate of 50 μmole/min, a Cp₂Mg flow rate of 0.5μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate of 10 slpm for9 sec. Subsequently a stop is implemented for 5 sec. without a change intemperature or pressure and an NH₃ flow rate of 4 slpm and an H₂ flowrate of 10 slpm. The preceding sequence is repeated 4 times to make 5pair period superlattice 1100.

With reference to table 2 in FIG. 18, and FIGS. 9a and 13, 5 pair shortperiod InGaN/AlGaN superlattice structure 1300 for distributed electronreflector 910 b may be may be deposited immediately on top of GaNtransition layer 429 if desired. Expected conditions for depositingIn_(0.1)Ga_(0.9)N:Mg layer 1301 with 0.001 μm thickness are atemperature of 800° C., a pressure of 700 Torr with a TEGa flow of 10μmole/min, a TMIn flow of 10 μmole/min, a Cp₂Mg flow rate of 0.2μmole/min, an NH₃ flow rate of 5 slpm for 15 sec. Subsequently a stop isimplemented for 10 sec. without a change in temperature or pressure andan NH₃ flow rate change from 5 to 4 slpm and an H₂ flow rate change from0 to 10 slpm. Expected conditions for depositing Al_(0.2)Ga_(0.8)N:Mglayer 1302 with a 0.003 μm thickness are a temperature of 800° C., apressure of 700 Torr with a TMGa flow rate of 34 μmole/min, a TMAl flowrate of 50 μmole/min, a Cp₂Mg flow rate of 0.5 μmole/min, an NH₃ flowrate of 4 slpm and an H₂ flow rate of 10 slpm for 9 sec. Subsequently astop is implemented for 5 sec. without a change in temperature orpressure and an NH₃ flow rate change from 4 to 5 slpm and an H₂ flowrate change from 10 to 0 slpm. The preceding sequence is repeated 4times to make 5 pair period superlattice 1300.

With reference to table 2 in FIG. 18, and FIGS. 9a and 15, 5 pair shortperiod InGaN/GaN superlattice structure 1500 for distributed electronreflector 910 b may be may be deposited immediately on top of GaNtransition layer 429 if desired. Expected conditions for depositingIn_(0.1)Ga_(0.9)N:Mg layer 1501 with 0.001 μm thickness are atemperature of 800° C., a pressure of 700 Torr with a TEGa flow of 10μmole/min, a TMIn flow of 10 μmole/min, a Cp₂Mg flow rate of 0.1μmole/min, and an NH₃ flow rate of 5 slpm for 15 sec. Subsequently astop is implemented for 5 sec. without a change in temperature orpressure and an NH₃ flow rate of 5 slpm. Expected conditions fordepositing GaN:Mg layer 1502 with a 0.002 μm thickness are a temperatureof 800° C., a pressure of 700 Torr with a TEGa flow rate of 10μmole/min, a Cp₂Mg flow rate of 0.1 μmole/min, and an NH₃ flow rate of 5for 30 sec. Subsequently a stop is implemented for 5 sec. without achange in temperature or pressure and an NH₃ flow rate of 5 slpm. Thepreceding sequence is repeated 4 times to make 5 pair periodsuperlattice 1500.

With reference to table 2 in FIG. 18 and FIG. 9a, high aluminum contenttunnel barrier layer 910 c may be deposited immediately on top of GaNtransition layer 429 if desired. Conditions for depositingAl_(0.2)Ga_(0.8)N:Mg tunnel barrier layer 910 c with 0.015 μm thicknessare a temperature of 900° C., a pressure of 700 Torr with a TMGa flowrate of 34 μmole/min, a TMAl flow rate of 58 μmole/min, a Cp₂Mg flowrate of 0.5 μmole/min, an NH₃ flow rate of 4 slpm and an H₂ flow rate of10 slpm for 40 sec.

While the invention has been described in, conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all other such alternatives, modifications, and variations thatfall within the spirit and scope of the appended claims.

What is claimed is:
 1. An asymmetric waveguide nitride laser diodestructure comprising: an active layer having a first and second surface;a transition layer less than approximately twenty nanometers thick incontact with said first surface of said active layer; a p-cladding layerdisposed adjacent to said transition layer; and an n-type layer incontact with said second surface of said active layer.
 2. The structureof claim 1 wherein said n-type layer is a waveguiding layer.
 3. Thestructure of claim 2 wherein said waveguide layer is comprised ofsilicon.
 4. The structure of claim 1 wherein said p-cladding layer iscomprised of magnesium.
 5. The structure of claim 1 wherein saidp-cladding layer is in contact with a cap layer acting as an ohmiccontact.
 6. The structure of claim 2 wherein an n-cladding layer is incontact with said waveguiding layer.
 7. The structure of claim 1 whereinsaid transition layer has a thickness less than 0.01 μm.
 8. Thestructure of claim 1 wherein said transition layer is undoped.
 9. Thestructure of claim 6 wherein said n-cladding layer comprises morealuminum than said p-cladding layer.
 10. The structure of claim 1wherein said p-cladding layer has a thickness less than 0.005 μm. 11.The asymmetric waveguide nitride laser diode structure of claim 1wherein the transition layer is less than six nanometers thick.
 12. Theasymmetric waveguide nitride laser diode structure of claim 1 whereinthe transition layer is undoped GaN.
 13. The asymmetric waveguidenitride laser diode structure of claim 1 wherein the transition layer isbetween two and six nanometers thick.